1. Field of the Invention
The present invention relates generally to the recycling of rubber products. More particularly, the present invention relates to methods and compositions wherein comminuted scrap rubber is incorporated into a polyurethane matrix to form a wide variety of useful materials and products.
2. Description of Related Art
The recycling of rubber products has received a great deal of attention in recent years. Typical rubber recycling processes involve grinding or otherwise comminuting scrap rubber product to form rubber particles. These rubber particles are recycled by mixing them with various different binders and molding or otherwise fabricating the rubber-binder mixture into desired articles. Typical uses for recycled rubber particles include the formation of various underlayments, surface coatings, tires, and other articles where a strong and resilient material is required. The binders which have been used to form the composites of recycled rubber particles include asphalt, synthetic and natural rubber, epoxies, and polyurethanes.
The majority of rubber recycling effort has been directed toward automobile and truck tires. The extremely large quantity of automobile and truck tires which is discarded each year requires that some degree of recycling occur. Processes used to recycle automobile and truck tires must necessarily be well suited for handling large quantities of recycled material. The process must be capable of taking large quantities of rubber particles directly from tire grinding and shredding equipment and transforming the particles into a useful material which can be molded or shaped into desired finished products.
Polyurethanes are popular binders which have been used to form composites with recycled rubber particles. A common procedure for forming a polyurethane involves mixing a polyisocyanate with a polyol and suitable catalyst. The resulting polyurethane will be a non-cellular material provided that a blowing agent is not present in the composition. Addition of a blowing agent causes the polyurethane to foam, resulting in a microcellular polyurethane material. Water, if present, can function as a blowing agent during polyurethane formation because the carbon dioxide generated by the reaction of water and isocyanate becomes trapped within the polyurethane matrix and produces a foam structure.
Polyurethane composite materials wherein recycled rubber particles are incorporated into a polyurethane are well suited for use in tires, shoe and boot soles, mattings, and other applications where a high degree of strength and wear resistance are required. However, recycled rubber particles may contain sufficient moisture to produce an excess of foaming, resulting in composites which are softer than desired.
An object of the present invention, therefore, is to provide a process in which rubber tires and other discarded rubber products can be incorporated into a polyurethane matrix in a simple and efficient manner to provide a polyurethane composite material which has adequate functional properties including high strength and wear resistance.
Another object of this invention is to control the extent of foaming in the production of polyurethane-rubber composites.